WASHINGTON – The Justice Department said Sunday that it is looking into the shooting death of Trayvon Martin to determine whether federal prosecutors will file criminal charges for possible civil rights violations now that George Zimmerman has been acquitted in the state case in Florida.

The department opened an investigation into 17-year-old Trayvon’s death last year but stepped aside to allow the state prosecution to proceed.

In a statement, the Justice Department said the criminal section of its Civil Rights Division, the FBI and the U.S. Attorney’s Office for the Middle District of Florida are continuing to evaluate the evidence generated during the federal probe, in addition to the evidence and testimony from the state trial.

Sunday, NAACP President Benjamin Todd Jealous started a petition calling for the Justice Department to open a civil rights case against Zimmerman in the shooting death of Trayvon, but experience has shown that it’s rarely easy getting convictions in such high-profile prosecutions.

“The Justice Department would face significant challenges in bringing a federal civil rights case against Mr. Zimmerman,” said Alan Vinegrad, the former U.S. attorney in the Eastern District of New York. “There are several factual and legal hurdles that federal prosecutors would have to overcome: They’d have to show not only that the attack was unjustified, but that Mr. Zimmerman attacked Mr. Martin because of his race and because he was using a public facility, the street.”

As to the last element, the confrontation between Zimmerman and the shooting victim occurred in a gated community, which may not fit the legal definition of a public facility.

Zimmerman’s acquittal left many Americans wondering Sunday how the justice system could allow him to walk away from the fatal shooting. But the essential criteria for deciding the case came from the court itself, which told jurors that Zimmerman was allowed to use deadly force when he shot the teenager not only if he actually faced death or bodily harm, but also if he merely thought he did. And jurors heard plenty of conflicting evidence and testimony that could have created reasonable doubt.

Some Martin family supporters may never understand the gap between the legal basis for the acquittal and what they perceived as the proper outcome: Zimmerman’s conviction for either second-degree murder or manslaughter.

“There is a difference between the law and what people think is fundamentally justice,” said Barbara Arnwine, president and executive director of the Lawyers’ Committee for Civil Rights Under Law, based in Washington.

Under Florida law, jurors were told to decide whether Zimmerman was justified in using deadly force by the circumstances he was under when he fired his handgun. The instructions they were given said they should take into account the physical capabilities of both Zimmerman, 29, and Trayvon. And if they had any reasonable doubt on whether Zimmerman, a neighborhood watch volunteer, was justified in using deadly force, they should find him not guilty.

“Beyond a reasonable doubt” is the highest standard of proof prosecutors face in American criminal courts. It means the jurors believe there is no other logical explanation for what happened than the defendant is guilty. If faced with two plausible explanations for what happened, jurors are supposed to acquit.

“The danger facing George Zimmerman need not have been actual; however, to justify the use of deadly force, the appearance must have been so real that a reasonably cautious and prudent person … would have believed the danger could be avoided only through the use of that force,” the instruction read.

Jurors refused to talk with reporters after the verdict about how they reached their decision Saturday night. Their names are being kept secret until Judge Debra S. Nelson lifts an order protecting their identities.

After the verdict, State Attorney Angela B. Corey said the use of deadly force is often one of the toughest areas of the law for prosecutors. Gov. Rick Scott appointed her office to the case a few weeks after the shooting when local prosecutors didn’t press charges. She said that when a victim shoots a suspected robber or rapist, the use of deadly force is clearly justifiable but that in cases such as Zimmerman’s, the lines get blurry.

“That’s why this case was unique, in a sense, and that’s why this case was difficult,” she said.

The acquittal touched off mostly peaceful demonstrations nationwide, with protesters calling it a miscarriage of justice. Church services and street protests ranging from a few dozen to hundreds of people decried Trayvon’s death as a tragedy made worse by a flawed verdict.

In New York City, protesters at a massive gathering in Union Square hoisted placards bearing Trayvon’s picture. In the Florida city where the trial took place, teens wearing shirts bearing his picture attended a youth service.

In Oakland, Calif., one demonstration ended in vandalism in the hours after Saturday’s verdict. A protest in Los Angeles was broken up by police firing beanbag rounds.